The present invention relates generally to a wide-angle zoom lens system, and more specifically to a wide-angle yet high-magnification zoom lens system which can have satisfactory performance with a simple lens arrangement.
Wide-angle zoom lenses so far available in the art are basically broken down into two types, one type being a negative power-preceding type two-group zoom lens having a negative-positive refracting power profile, as viewed in order from an object side thereof, and another type being a positive power-preceding type three-group zoom lens having a positive-negative-positive refracting power profile, as viewed in order from an object side thereof. There is also proposed an extension or modification of these two basic types, wherein lens groups are divided for the achievement of high magnifications and large aperture ratios.
The negative power-preceding type has a retrofocus type arrangement at the wide-angle end, and is characterized in that the back focus can be reasonably ensured. Since the first lens group has negative refracting power, on the one hand, the height of incidence of axial rays on the rear lens group tends to increase at the telephoto end in particular, making aberration correction difficult. Thus, the negative power-preceding type has a problem in conjunction with the achievement of high zoom ratios and large aperture ratios. This type is also characterized in that the rear lens group has zooming action, the first lens group makes a nonlinear movement, and the spacing between the lens groups becomes narrow at the telephoto side. JP-A 53-140047 shows a zoom lens having an angle of view of about 74.degree. and a zoom ratio of 2 or more, constructed with two lens groups, positive and negative. This zoom lens is used together with a flare stop to restrict ray bundles. JP-A 53-43539 shows that a zoom ratio of about 3 is achievable with four lens groups, negative, positive, negative and positive. However, there is some limit to the zoom ratio achieved. This publication discloses a unique variation of dividing the first lens group for the purpose of obtaining large aperture ratios rather than high zoom ratios.
On the other hand, the positive power-preceding type uses a zooming mode where the movement of each lens group takes part in zooming action. Attempts have been made to achieve size reductions by increasing the refracting power of the second lens group and increasing the refracting power of the first lens group accordingly. To permit this type to have a high zoom ratio, the third lens group is often divided into subgroups, thereby making correction for field curvature. Nonetheless, an increase in the diameter of the front lens and complexity of the lens arrangement still unavoidably occur. JP-A 54-25747 discloses a wide-angle to quasi-telephoto zoom lens, and JP-A 55-62419 discloses a wide-angle yet high-magnification zoom lens reaching completion. These have become a paradigm of currently available zoom lenses. In view of lens arrangements, however, complexity problems remain unsolved.